A well-known diving toy called a "Cartesian diver" is essentially an inverted cup placed in a bath of water. A small weight is attached to the cup lip to achieve neutral buoyancy. The weight also ensures the center of mass is lower than the center of buoyancy, so the cup will not invert.
In operation, the diver sinks to a depth determined primarily by air pressure. If the air pressure above the liquid increases, this increased pressure is transmitted by the fluid to the air trapped inside the diver. The external pressure squeezes on the trapped air, reducing its volume. This reduces the buoyancy of the diver, and it sinks. As the diver sinks, pressure from the weight of the column of water above the diver increases, squeezing on the trapped air in the diver. Although the pressure increases linearly with depth, the air in the diver "stiffly" resists compression according to the ideal gas law. Eventually, it reaches a depth where the air pressure inside the diver is equal to the pressure in the water adjacent to the lip edge. The diver hovers at this depth. Similarly, if the external air pressure is reduced, the diver rises. Thus, one can think of the diver as a simple barometer.
The difficulty with the Cartesian diver as a toy to amuse or teach is its lack of dynamism. Atmospheric temperature changes but slowly so the diver also moves slowly. Accordingly there is a need for a more dynamic diving toy.